Introduce channel attr to Order model

Could be set when importing orders from other storefronts

Conflicts:
	api/CHANGELOG.md
	api/spec/controllers/spree/api/orders_controller_spec.rb
	core/CHANGELOG.md
Add `ship_total` to api order_attributes
Make api helper attributes class attributes

To make it easier to append more attributes. Other applications /
extensions can append their own stuff without having to override
methods. e.g.

        Spree::Api::ApiHelpers.order_attributes.push :locked_at
Make OrderInventory aware of `track_inventory_levels`

It should not create backordered inventory units when it is set `false`
use the new Spree logo [Fixes #3975]
Replaced Taxon :position refs with :child_index=

Fixes #3390
Fixes #3956

Taxon position unnecessarily fights against the awesome_nested_set
API. By using a_n_s's :move_to_child_with_index method we can more
fully leverage acts_as_nested_set.

Conflicts:
	core/app/models/spree/taxon.rb
	core/spree_core.gemspec